This invention relates to an apparatus for monitoring degradation of insulation of an electrical installation and more particularly it relates to an apparatus for monitoring degradation of insulation of an electrical installation that can identify one or more than one defectively insulated cables of a power transmission line or electrical equipments.
The insulation of a power cable and the equipment connected to thereto can become locally defective due to various causes.
While possible causes of faulty insulation include external mechanical forces, chemical changes of the insulator and so called water and/or electric trees, approximately eighty percent of serious insulation troubles are attributable to degraded insulation and therefore a variety of methods have been proposed for assessing the conditions of insulation of various items. A water tree mentioned above is a kind of deterioration of the insulator caused by water in an electric field, bringing about a destruction in the insulator that spreads like branches of a tree. An electric tree is also a kind of deterioration of the insulator which is caused to take place in a sectional high electric field in an inner part of the cable insulator or on a border surface between the semi-conductor layer and the insulator, wherein a sectional destruction occurs in the first place which is then spread like branches of a tree.
According to one of those proposed methods, the power supply system is periodically tested for the conditions of insulation thereof by temporarily suspending the power supply. In this method, the test may be conducted, for example, by applying DC voltage to the power line. What are measured here are: firstly, the partial discharge, secondly, the dielectric relaxation that can be assessed by the residual voltage, discharge current or residual charge, and, thirdly, the potential damping and leakage current that affect the performance of the insulator.
Alternatively, the test may be conducted by applying AC voltage to the power line. What are determined by this alternative test includes the partial discharge and the dielectric relaxation to be assessed by the dielectric tangent.
Apart from this, there is a method wherein a power supply system is examined under hot-line conditions by measuring the insulation resistance or the distributed DC current with a portable or stationary measuring instrument.
The above described method of assessment of the insulation performance of a power supply system involving periodical suspension of power supply, however, is very time consuming because all the power lines constituting the system have to be examined one after another. Moreover, the number of spots to be tested during an interruption of power supply will be limited. All these circumstances hinder thorough examination of the insulation of a power supply system for determining potential faults and consequently introduction of any preventive measures.
On the other hand, the method of examining a power supply system under hot-line conditions with a portable measuring instrument is accompanied by a problem of requiring skilled engineers for carrying out the examination as it involves sophisticated operation of insulating the neutral point of the grounding type transformer for the portable instrument from the earth for direct current. The operation is labor intensive and requires elaboration on the side of engineers for safety purposes. Besides, the insulation of the power supply system cannot be monitored on continuous basis.
When a stationary measuring instrument is used for this method of examining a power supply system under hot-line conditions, the grounding point of the cable shield needs to be insulated from the ground for direct current and therefore the insulation of the power supply system cannot be monitored on continuous basis either.
Moreover, the above described methods for examining and detecting degraded insulators of a power supply system under hot-line conditions are applicable only to high voltage cables and cannot detect local problems such as void discharge in an insulator.